The present invention relates to a single phase unidirectional transducer and a SAW (surface acoustic wave) filter comprising such a transducer.
A single phase unidirectional transducer and surface acoustic filter comprising such transducer are disclosed in JP-B-62-31860 and U.S. Pat. No. 5,703,427, for example.
According to the above-mentioned SAW filter comprising a single phase unidirectional transducer, it is possible to improve the insertion loss substantially by remarkably increasing the unidirectional property based on an asymmetric electrode structure and by properly utilizing an excitation effect and reflection effect of electrode fingers on the electrode structure.
However, due to the increased unidirectional property, the SAW filter comprising such single phase unidirectional transducer gives rise to a tendency to degrade the attenuation property of the filter and increase the ripple of the filter. In order to overcome the degradation of the attenuation property and the increase of the ripple, the single phase unidirectional transducer as shown in FIG. 1 is disclosed in 1994 URTRASONIC SYMPOSIUM-1, xe2x80x9cA New Concept in SPUDT Design: the RSPUDT (Resonant SPUDT)*xe2x80x9d by P. Venturea, M. Solal, P. Dufilixc3xa9**, J. M. Hodxc3xa9 and F. Roux.xe2x80x9d
The unidirectional transducer disclosed by Ventura et al. includes a rectangular quartz substrate 1 as a piezoelectric substrate, a positive electrode 2 formed on the quartz substrate 1 and a negative electrode 3 formed on the quartz substrate 1. The positive electrode 2 comprises a plurality of electrode fingers 4, 5 each having a width xcex/8 as measured in the propagation direction of the fundamental surface acoustic wave; wherein xcex represents a propagation wavelength of the fundamental surface acoustic wave. The negative electrode 3 comprises a plurality of electrode fingers 6, 7, 8 each having a width xcex/8 as measured in the propagation direction of the surface acoustic wave and a plurality of electrode fingers 9, 10 each having a width 3xcex/8 as measured in the propagation direction of the surface acoustic wave.
The electrode fingers of the positive electrode 2 and the negative electrode 3 constitute a first group A generating a combined wave of an excited wave and a reflected wave with the propagation direction toward the right side in FIG. 1, a second group B generating a combined wave of an excited wave and a reflected wave with the propagation direction toward the left side in FIG. 1, and a third group C serving to match the phase of the combined wave generated in the first group A and the phase of the combined wave generated in the second group B. The first group A is constituted by electrode fingers 9, 4 and 6, the second group B is constituted by electrode fingers 8, 5 and 10, and the third group C is constituted by electrode finger 7.
If such a unidirectional transducer is used as an input transducer, for example, in the SAW filter, the number of the first groups A is equal to or larger than that of the second groups B. As the surface acoustic wave generated in the first groups A is combined with the surface acoustic wave generated in the second groups B and these surface acoustic waves influence one another, a weighting of the reflection and the excitation of the surface acoustic wave is performed. It is thus possible to improve the attenuation property and reduce the ripple by adjusting the surface acoustic wave generated in the first group A.
However, in the case of the unidirectional transducer with an electrode structure as shown in FIG. 1, the unidirectional property is so remarkable that only a rough adjustment of the surface acoustic wave can be performed. This makes it difficult to control the surface acoustic wave and achieve a substantial reduction of the ripple of the SAW filter.
It is an object of the present invention to provide single phase unidirectional transducer and a SAW filter comprising such transducer which is effective in achieving the substantial reduction of the ripple.
According to the present invention, there is provided a unidirectional transducer for a SAW filter comprising: a piezoelectric substrate and a first electrode formed on the substrate. The first electrode comprises a plurality of electrode fingers, the electrode fingers each having a width about xcex/8 in a propagation direction of a fundamental surface acoustic wave with a propagation wavelength xcex.
A second electrode is formed on the substrate so as to be substantially parallel to the first electrode, and comprises a plurality of first and second electrode fingers, the first electrode fingers each having a width about xcex/8 in the propagation direction of the fundamental surface acoustic wave, and the second electrode fingers each having a width about 3xcex/8 in the propagation direction of the fundamental surface acoustic wave.
A first group comprises one second electrode finger, one electrode finger arranged at a center distance 3xcex/8 in a direction opposite to the propagation direction from the one second electrode finger, and one first electrode finger arranged at a center distance 5xcex/8 in a direction opposite to the propagation direction from the one second electrode finger.
A second group comprises another second electrode finger arranged at center distance 10xcex/8 in the propagation direction from the one second electrode finger in the first group, another electrode finger arranged at a center distance 13xcex/8 in the propagation direction from the one second electrode finger in the first group, and another first electrode finger arranged at a center distance 15xcex/8 in the propagation direction from the one second electrode finger in the first group.
A third group comprises three first electrode fingers arranged at center distances of 3xcex/8, 5xcex/8 and 7xcex/8, respectively, in the propagation direction from the one second electrode finger in the first group.
Traditionally, it was thought that a weighting of the reflection and the excitation of the surface acoustic wave is not performed if the transducer with an electrode structure constituted by the first, the second and the third groups is used in the SAW filter because the surface acoustic wave generated in the first group propagates at the direction opposite to the direction toward the second group, on one hand, and the surface acoustic wave generated in the second group propagates at the direction opposite to the direction toward the first direction, on the other hand, so that these surface acoustic waves do not influence to one another.
However, in fact, the major part of the surface acoustic wave generated in the first group propagates at the direction opposite to the direction toward the second group and the minor part of the surface acoustic wave generated in the first group propagates toward the second group. Similarly, the major part of the surface acoustic wave generated in the second group propagates at the direction opposite to the direction toward the first group and the minor part of the surface acoustic wave generated in the second group propagates toward the first group. As a result, the surface acoustic wave generated in the first group and propagating at the direction opposite to the direction toward the second group is influenced by the surface acoustic wave propagating toward the first group, and the surface acoustic wave generated in the second group and propagating at the direction opposite to the direction toward the first group is influenced by the surface acoustic wave propagating toward the second group.
Therefore, it is possible to adjust the surface acoustic wave finely, control the surface acoustic wave easily and achieve substantial reduction of the ripple easily. In this case, three electrode fingers in the third group interposed between the first group and the second group serve to match the phase of the surface acoustic wave generated in the first group to the phase of the surface acoustic wave generated in the second group.
If the unidirectional transducer according to the present invention is used as the input transducer in the SAW filter, the number of the second groups is usually equal to or larger than that of the first groups. On the other hand, if the unidirectional transducer according to the present invention is used as the output transducer in the SAW filter, the number of the first groups is usually equal to or larger than that of the second groups. The unidirectional transducer according to the present invention, in addition to the first, second and third groups discussed above, can be formed to incorporate other electrode structures. It is also possible to perform thinning of the electrode structure.
According to the present invention, there is provided a SAW filter comprising: a substrate, an input transducer formed on the substrate, in which an electric signal is input from external and in which the electrical signal is transformed into a surface acoustic wave, and an output transducer formed on the substrate, in which the surface acoustic wave excited in the input transducer is transformed into the electric signal, and in which the electric signal is output to external.
At least one of the input and output transducers comprises a unidirectional transducer. The transducer comprises a first electrode formed on the substrate comprising a plurality of electrode fingers, the electrode fingers each having a width about xcex/8 in a propagation direction of a fundamental surface acoustic wave with a propagation wavelength xcex.
A second electrode is formed on the substrate so as to be substantially parallel to the first electrode, and comprises a plurality of first and second electrode fingers, the first electrode fingers each having a width about xcex/8 in a propagation direction of the fundamental surface acoustic wave, and the second electrode fingers each having a width of about 3xcex/8 in the propagation direction of the fundamental surface acoustic wave.
A first group comprises one second electrode finger, one electrode finger arranged at a center distance 3xcex/8 in a direction opposite to the propagation direction from the one second electrode finger, and one first electrode finger arranged at a center distance 5xcex/8 in a direction opposite to the propagation direction from the one second electrode finger.
A second group comprises another second electrode finger arranged at a center distance 10xcex/8 in the propagation direction from the one second electrode finger in the first group, another electrode finger arranged at a center distance 13xcex/8 in the propagation direction from the one second electrode finger in the first group, and another first electrode finger arranged at a center distance 15xcex/8 in the propagation direction from the one second electrode finger in the first group.
A third group comprises three first electrode fingers arranged at center distances of 3xcex/8, 5xcex/8 and 7xcex/8, respectively, in the propagation direction from the one second electrode finger in the first group.
According to the present invention, it is possible to achieve a substantial reduction of the ripple of the SAW filter.